Polyglutamine-Expanded Androgen Receptor Alteration of Skeletal Muscle Homeostasis and Myonuclear Aggregation Are Affected by Sex, Age and Muscle Metabolism.

Mathilde Chivet,Sara Parodi,Maria Josè Polanco,Elena Pegoraro,Maria Pennuto,Maurizio D’Antonio,Fabio Sambataro,Carlo Rinaldi,Marco Pirazzini,Marco Sandri,Emanuela Zuccaro,Udai Bhan Pandey,Manuela Basso,Vanina Romanello,Gianni Soraru,Chiara Scaramuzzino,Diana Piol,Caterina Marchioretti

doi:10.3390/cells9020325

Abstract

Polyglutamine (polyQ) expansions in the androgen receptor (AR) gene cause spinal and bulbar muscular atrophy (SBMA), a neuromuscular disease characterized by lower motor neuron (MN) loss and skeletal muscle atrophy, with an unknown mechanism. We generated new mouse models of SBMA for constitutive and inducible expression of mutant AR and performed biochemical, histological and functional analyses of phenotype. We show that polyQ-expanded AR causes motor dysfunction, premature death, IIb-to-IIa/IIx fiber-type change, glycolytic-to-oxidative fiber-type switching, upregulation of atrogenes and autophagy genes and mitochondrial dysfunction in skeletal muscle, together with signs of muscle denervation at late stage of disease. PolyQ expansions in the AR resulted in nuclear enrichment. Within the nucleus, mutant AR formed 2% sodium dodecyl sulfate (SDS)-resistant aggregates and inclusion bodies in myofibers, but not spinal cord and brainstem, in a process exacerbated by age and sex. Finally, we found that two-week induction of expression of polyQ-expanded AR in adult mice was sufficient to cause premature death, body weight loss and muscle atrophy, but not aggregation, metabolic alterations, motor coordination and fiber-type switch, indicating that expression of the disease protein in the adulthood is sufficient to recapitulate several, but not all SBMA manifestations in mice. These results imply that chronic expression of polyQ-expanded AR, i.e. during development and prepuberty, is key to induce the full SBMA muscle pathology observed in patients. Our data support a model whereby chronic expression of polyQ-expanded AR triggers muscle atrophy through toxic (neomorphic) gain of function mechanisms distinct from normal (hypermorphic) gain of function mechanisms.

Highlights

Spinal and bulbar muscular atrophy (SBMA), known as Kennedy’s disease, is an X-linked neuromuscular disease characterized by the selective dysfunction and degeneration of brainstem and spinal cord motor neurons (MNs) [1]
AR24Q mice suffered from severe hemorrhoids and developed difficulties to move in the cage, which required premature sacrifice, thereby resulting in a significantly reduced survival with a median of 26 weeks (χ2 Log−Rank = 16.03, p < 0.0001 relative to WT) (Figure 1B)
By measuring mitochondrial membrane potential in myofibers isolated from the flexor digitorum brevis (FDB) muscle upon treatment with the F1 F0 -ATPase blocker, oligomycin, we found that the number of fibers with mitochondria depolarized by oligomycin was significantly (p < 0.05) increased in 8- and not

Summary

Introduction

Spinal and bulbar muscular atrophy (SBMA), known as Kennedy’s disease, is an X-linked neuromuscular disease characterized by the selective dysfunction and degeneration of brainstem and spinal cord motor neurons (MNs) [1]. Patients present with a wide array of peripheral organ and tissue dysfunction, including skeletal muscle weakness, wasting and atrophy [2,3]. SBMA is caused by exonic expansions of a CAG tandem repeat, resulting in an aberrantly elongated polyglutamine (polyQ) tract in the androgen receptor (AR) gene [4]. SBMA is triggered by the binding of polyQ-expanded AR to androgens and this is the reason why the disease fully manifests in males [6,7]. Patients show mild signs of androgen insensitivity and endocrine abnormalities, thereby implicating that the loss of AR function (LOF) contributes to disease [11]. The androgen-dependent nature of the disease and experimental evidence support chemical and physical castration as a therapeutic strategy for SBMA [6,7,10], clinical trials based on this approach showed benefits only in a subset of patients [12]

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